I'm using the MX500 on XP with Firebird 0.7 and all of the buttons are working. I never installed any mouse-specific drivers, the buttons all worked as advertised, so no need. In any event, it would seem that it has everything to do with the cordless version.. I do have my mouse hooked up using the USB connector.

My mouse wheel scrolls up and down.
If I press my mouse wheel and let go, I can move the mouse all around to scroll the page.
If I press the thumb buttons the site will page forward and backward as clicked.
The buttons that are on top and bottom of the mouse wheel function as scrolling up and down.